Undersea optical communication systems include land-based terminals containing transmitters and receivers connected by a cabled-fiber-transmission medium that includes periodically spaced repeaters, which contain optical amplifiers whose purpose is to compensate for the optical attenuation in the cabled-fiber. As the repeaters are usually placed undersea and away from power sources, power must be supplied remotely to the repeaters. The cabled-fiber therefore usually contains a copper conductor to carry electrical power to the repeaters from the terminals. These undersea systems serve to carry optical communication signals (i.e., traffic) between the terminals. The traffic on these systems can consist of voice, data, television, Internet traffic, international telephone traffic, etc. Consequently, the revenue lost when the system is down can be significant. Therefore, these systems must have high reliability and availability.
To provide increased flexibility in undersea network architecture beyond simple point-to-point interconnection between land-based terminals, a branching unit is provided, which allows traffic to be split or switched to/from multiple landing points. Conventional branching units typically manage the cabled-fiber interconnections and the power conductor paths among three cables. The latter is necessary to maintain as much traffic carrying capability when a fault occurs in one of the three cable legs, which increases the availability of the system. FIG. 1 shows a simplified schematic diagram of a conventional branching unit 10. The branching unit 10 is a three port device in which each of the ports 12, 14 and 16 receive a cabled-fiber 20, 22, and 24, respectively. Each of the cabled-fibers 20, 22, and 24 includes one or more optical fibers 26 and an electrical power conductor 28. Cabled-fibers 20 and 22 are generally referred to as trunks and cabled-fiber 24 is generally referred to as a branch. Branching unit 10 can drop incoming traffic on trunks 20 and/or trunk 22 to branch 24 and add traffic from branch 24 to trunks 20 and/or trunk 22.
A power-switched branching unit such as shown in FIG. 1 is generally configured to allow re-routing of electrical power from the terminals in the presence of a fault in one of the cables, so that two of the three cable legs in a branched system can still be powered. Such a power-switched branching unit usually has three operating states: normal, alternate-normal, and grounded-trunk. The power-switched branching unit can be configured in any of these three states by the appropriate power-up sequencing from the terminals of the three legs. The reconfiguration of the branching unit is typically performed by a relatively complex series of relays, which significantly adds to the cost and complexity of the device.
Co-pending U.S. patent application Ser. No. 10/687,544 discloses a simplified branching unit in which electrical power is only provided to two of its legs. The third leg employs a cable that does not incorporate an electrical conductor and hence serves as an unrepeatered leg. Because only two legs are powered instead of three, the complex power-switching arrangement required in conventional branching units may be eliminated. FIG. 2 shows a simplified schematic diagram of the branching unit 30 depicted in the aforementioned patent application. The branching unit 30 is a three port device in which each of the ports 46, 47 and 48 receive a cabled-fiber 32, 34 and 44 respectively. Cabled-fibers 32 and 34 serve as cable trunks that each includes one or more optical fibers 40 and an electrical power conductor 38. Cabled-fiber 44 includes one or more optical fibers 36 and, as shown, does not include an electrical conductor.
While the aforementioned branching unit is sufficient when one leg, the branch, is unrepeatered, it would be desirable to provide an adjunct device for use in connection with the branching unit that allows the branch to be repeatered when so desired.